The invention relates to an automatic storage and stacking device for sheets of laminar material, such as corrugated cardboard or the like, comprising a transportation unit which removes the sheets of laminar material from a cutting group, overlaps them and conveys them towards a storage surface, said transportation unit comprising a plurality of conveyors in series, which can be operated at varying speeds so as to allow separation of a first series of sheets from a successive second series of sheets.
Storage devices of this type are commonly used to remove sheets of stiff corrugated cardboard produced by a cutting unit supplied with a continuous strip of cardboard. These storage devices position the cut sheets in a scale-like arrangement, i.e., so that they partially overlap each other, along a plurality of conveyors in series. At the end of the line of conveyors, the sheets are stored and stacked on a roller table, storage surface or other means. When the stack of sheets is complete, the supply of sheets must be temporarily interrupted, the full storage surface replaced by an empty one and the sheets then supplied again onto the storage surface. In order to perform these operations without stopping the upstream processing line, namely the line for producing and cutting the cardboard sheets and effecting longitudinal ribbing and transverse cutting, use is currently made of conveyor-type storage devices which, by suitably varying the feed speed of the individual conveyors, make it possible to create an interruption in the line of sheets which are in a scale-like and partially overlapped arrangement. The speed variation sequence is such that it is possible to accelerate unloading of the last sheet of a first series of sheets to be stored on the storage surface during filling, at the same time slowing down temporarily the supply of the first sheet of the successive second series of sheets to be stacked on the new storage surface. The interruption is long enough to allow the storage surfaces to be changed over. Plants of this type are known from the prior art and are described, interalia in the U.S. Pat. No. 4,313,600.
In order to slow down the first sheet of the new series and thus ensure continuity in the supply of the sheets, various systems have been proposed for raising temporarily the end of the first sheet of the new series from the transportation unit. Among other things, it has been proposed to hold the end of said sheet using a suction system arranged in the region of one of the conveyors of the transportation line. These known systems have proved to be unreliable since they do not always manage to support the sheet of laminar material or retain it, in particular when the laminar material is buckled or its surface is not smooth, which is often the case on account of the nature of the product.
An object of the present invention is to provide an automatic storage and stacking device of the above-mentioned type which overcomes the drawbacks described above and ensures effective and reliable temporary holding of the rear edge, that is the end, of the first sheet of each series of sheets to be stacked on each storage surface.
These and other objects and advantages, which will become clear to persons skilled in the art upon reading the text which follows, are achieved with an automatic storage and stacking device of the above-mentioned type, characterized in that with one of said conveyors there is associated a retaining means for mechanically holding the rear portion of the first sheet of said second series, which retaining means comprises a gripper member able to perform an alternating movement along the conveyor with which said gripper member is associated, so as to follow the feed movement of the sheet.
According to a particularly advantageous embodiment, during the forward travel in the direction of movement of the conveyor with which it is associated, said gripper member advances at a speed slower than the feed speed of the conveyor with which it is associated and equal to the feed speed of the upstream conveyor.
According to a possible embodiment, the gripper member comprises a carriage accommodating a bar movable with respect to the conveyor with which said gripper member is associated and, above said bar, a member oscillating between an inactive position and a position in which it cooperates with said bar so as to clamp the sheet of laminar material between said bar and said oscillating member.
Advantageously, the conveyor with which the gripper member is associated may consist of a plurality of parallel belts, and the bar may have a plurality of opposition means, each of which extends between two adjacent belts of said conveyor.